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Herring larvae cultured in Bergen (W Norway), fed natural zooplankton (copepods) at first feeding, were found to harbour immature trematodes in the dorsal body musculature. These parasites were identified as larval didymozoids. The metacercariae are small, 365-567 mm, and of the Monilicaecum type, with an almost constant number of caecal chambers, 9 per caecum. In the forebody there is a region packed with glandular cells, surrounding the posterior oesophagus and the caecal bifurcation, the anterior part consists of agranular cells and posterior part consists of granular cells. There is no dilated region of the oesophagus ("stomach"). The morphology of these metacercariae is compared to previously described forms. The only common definitive host of didymozoids in Norwegian inshore waters is the mackerel (Scomber scombrus L.), which is the probable final host. The morphology of the metacercariae is discussed in relation to Halvorsenius exilis and Nematobothrium spp., adult didymozoids infecting the mackerel.